Mike,
As others have said, just log it twice, once for each grid.
The portable and handheld designations are commonly used on
satellites. Portable can have many meanings. Some use it
to say they are using an HT or operating on batteries.
Originally, the portable designator was used to indicate you
were not at your home QTH.
In the 60's and earlier, the FCC was much more controlling
of amateur radio than they are now. If you were not at home,
you were obliged to sign /mobile or /portable on voice or
on CW, /8 if you were in the 8th call area etc. If you moved
out of your call area, you had to get a new callsign with the
proper number for your new location.
If you were to be away from your home QTH for more than 72 hours,
you had to tell the FCC Engineer in Charge of your district where
you were going to be operating from. If on a trip, you had to
supply an itinerary telling them where you would be on a certain
day. The FCC wanted to know where the transmitters were and who
was operating them.
Another requirement was to log every contact. If the FCC came
calling, you had better have your logbook available. There was
a requirement that you operate so many hours in a certain period
of time in order to renew your license. In the early days, the
FCC was feared. There were lots of rules to follow. A mistake
could cost you your license.
Today things are much different and the rules are very friendly.
But signing portable still has meaning when it indicates you are not at
home and possibly operating in another grid. When you hear "portable,"
it gives a hint that you might want to check your grid list to see
if it's a new one. It also might influence you to give priority to
the "portable" station since it may be operating under less than
desirable circumstances, out in the open, on the side of a road, out
in the rain etc., for the primary purpose of handing out a new grid
for those who are paying attention in their comfortable hamshack.
Of course, many use "portable" when they are at home with an HT. If you
hear me signing portable, you'll know I'm not at home. Time to check your
grid list.
73,
John K8YSE